Heretofore, various methods have been used to shield electronic equipment. Metallic boxes and cans fabricated from steel, copper, aluminum, etc., were used by surrounding high EMI emitters for shielding. However, because shields fabricated from metal were cumbersome, heavy and costly the electronics industry has resorted to metallized plating on plastics. But, the results obtained with metallic coatings were not always satisfactory. In addition to being relatively non-economic, once such metallic coatings were scratched through they would lose part of their shielding efficiency. Unless such conductive coatings are continuous and free of voids, electromagnetic waves will be free to pass through. Frequently, it was difficult to obtain a dependable, 100% effective coating which was also resistant to peeling.
Further efforts by the electronics industry to develop more dependable light-weight materials for EMI shielding has led to a third approach, namely electrically conductive composites consisting of a variety of polymers loaded with conductive fillers and/or reinforcements. It was anticipated that intricate shapes could be moded from the composite materials by a conventional means, yielding a finished part that promised to be more economic and dependable than metal or metal-coated plastics.
One of the most common conductive fillers used in the composite approach has been carbon black. The major advantage of resinous composites containing this filler has been that it flows readily in molding processes, and therefore, provides a high degree of design flexibility. However, in order to obtain nominally acceptable shielding efficiencies to EMI emissions loadings of carbon black in molding compounds should be above 15% by weight. A similar requirement also exists in the case of graphite powder. In each instance, the loading levels required of powdered and spherical-shaped conductive fillers has been too high to achieve acceptable conductivity without reducing the impact resistance and other mechanical properties needed for EMI shields and housings. Carbon/graphite fibers also provide acceptable EMI shielding efficiencies, but because of very high cost they are not viewed as acceptable alternatives to the more economic powders and nodules.
Metal particles of various sizes and shapes such as aluminum, copper, nickel, zinc, etc., have also been used in molding compositions as conductive fillers. The principle factor influencing the performance of metalfilled composites is the aspect ratio of the particles. For example, spherical-shaped particles must be loaded to a 38% by volume concentration to achieve electrical conductivity. However, this frequently leads to poor mechanical properties and poor cost effectiveness. In contrast, fibrous metal particles are able to impart electrical conductivity to composites with as little as 7% by-volume metal. However, high aspect ratio fibers are difficult to process in that they become entangled and agglomerate producing a poorly dispersed mixture.
EU-A No. 1 0043 040 discloses composite materials for shielding against electromagnetic radiation containing in a polymer matrix metallized textile fabrics, filaments or metallized particles. The matrix material is a polymer plastic, such as e.g. a plasticized polyvinyl chloride. However, it is difficult to incorporate into a plastic matrix metallized fabrics and it is not possible to process the same in conventional manner by extrusion or injection molding when in granular form. It is difficult to distribute metallized filaments into a plastic matrix in the case of extrusion or injection molding, so that "windows" transparent to electromagnetic radiation are formed. Apart form the electrically conductive component, these materials also contain additives for increasing the electrical and/or magnetic conductivity, such as carbon black, graphite or ferrites. In the case of materials containing metallized filaments, they only have a limited shielding action and is in a range of 38 to 29 dB in the case of a frequency range of 50 MHz to 35 GHz.
DE-OS No. 35 18 335 also discloses an electromagnetic shielding material, which contains an electrically conductive carbon powder and a Mn-Zn-ferrite powder in a high molecular weight, organic matrix, such as rubber, thermoplastic or synthetic resin. However, the shielding capacity of this material only extends up to 45dB in the case of a standard frequency of 1000 MHz. In addition, the necessary ferrite filling quantities of 30 to 70 vol. % are so large, that the mechanical characteristics of the plastic materials are impaired.
Finally, EU-A No. 20 0 90 432 discloses an electromagnetic shielding material, in which short metal filaments with high electrical conductivity and a ferromagnetic component are present in a plastic matrix. The conductive metal filaments can be made form precious metals, copper, aluminium, zinc, nickel, iron and the alloys thereof, whilst the ferromagnetic material comprises ferrites, iron, cobalt and nickel. Apart from carbon black, a ferroelectric component can be present, such as barium titanate, lead titanate, lead niobate and lead zirconate. However, these shielding materials also suffer form the disadvantage of being difficult to process as a result of the metal filaments. The shielding action is also inadequate, being in the range 10 to 20 dB at 8 to 13 GHz.
The problem of the present invention is therefore to disclose an electromagnetic shielding material, which can easily be processed by conventional methods, particularly plastic extrusion and injection molding, whilst inhibiting "windows" which are transparent to electromagnetic waves and permitting an extremely good shielding action.